# Implementing a current sensing feature on LM2596 with negative output voltage

#### jsyschan

Joined Apr 28, 2012
12
I'm trying to add a current-limiting feature to my negative-output buck voltage circuit. My schematic is shown below. I'm using an LM2596 IC, and I'm attempting to get -9.5V out from a 15 V source. I saw a video of someone implementing a current limiting feature using the positive configuration, but I don't know how to do successfully accomplish that with the negative configuration. Right now, the circuit below maintains its negative output (with a small amount of ripple voltage) given various loads. However, if I add a 1 ohm sense resistor in the circuit, say before the first switch, the output varies. How should I go about implementing a current limiting feature with this negative voltage configuration?

#### crutschow

Joined Mar 14, 2008
29,785
If you can post the circuit schematic for the positive limit, that would be helpful.

What is the current-limit value you want, and if adjustable, over what range?

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#### jsyschan

Joined Apr 28, 2012
12
If you can post the circuit schematic for the positive limit, that would be helpful.

What is the current=limit value you want, and if adjustable, over what range?
In my end application, the max load is anticipated to be 500 mA. However, I'm designing this assuming a max limit of 1 amp in order to allow for some tolerance and to allow some more current to drive other loads such as an LED and whatnot. You can see the switches and the resistors on the right, which represents a changing load as the switches open/close.

Below is my circuit for the positive reference along with a simulation I ran. Top plot shows the voltage output of 5 V while the bottom half shows the current through the sense resistor and the corresponding voltage output of the op-amp, showcasing how the current measurement is translated into a voltage. Note how the feedback loop of the LM2596 includes that sense resistor to maintain its output voltage. In the negative configuration, the feedback is taken at ground, so I'm not sure how to start with it.

With regards to the current limit value, it's set by the second op-amp comparator, the voltage source at the negative input. Ideally, the current should handle a range over 0-1A, but say I set the limits for a lower limit, like 0.75 amps, I change the voltage source, the comparator will swing high and adjust the feedback voltage of the LM2596, which would lower the output voltage in order to maintain that max current limit.

This is just one part of a project I'm working on, designing a voltage supply for a specific application. At first, I was using linear regulators, but due to the high current and wide differential voltage, that didn't seem to work out well, so I'm using buck converters instead.